This invention relates generally to digital radio data communication systems and methods and, more particularly, to link adaptation techniques for selecting an optimum channel coding for a particular radio channel quality.
In digital radio systems the information is typically transmitted in data packets using a particular type of channel coding. The goal of channel coding is to improve the transmission quality when the transmitted signal encounters radio channel disturbances. When a data packet is coded, some amount of redundant information is added to the source data. A subsequent decoding operation then makes use of the redundant information to detect and/or correct bit errors that occurred during the transmission.
When the amount of redundant information is increased the error correction capabilities of the decoder can be improved. However, the improvement in error correction capabilities comes at the cost of a reduced net bit rate. As can be appreciated, it would be advantageous to use a robust channel coding technique during poor radio propagation conditions, and a channel coding technique having a high net bit rate when the radio channel conditions are good.
It is known in the art to employ link adaptation algorithms in an attempt to select an optimum channel coding technique for a given radio link. The selection of a particular channel coding technique is based on link or channel quality estimates.
In a typical link adaptation algorithm the receiver measures a carrier to interference ratio (C/I) or a raw Bit Error Rate (BER), and then makes an estimate of the channel quality by averaging the measurements over some period of time. Reference in this regard can be had to a publication entitled xe2x80x9cBit Error Rate Based Link Adaptation for GSMxe2x80x9d, J. Pons and J. Dunlop, Proceedings of the 1998 9th IEEE International Symposium on Personal, Indoor and Mobile Radio Communicationsxe2x80x9d, PIMRC, Part 3, pages 1530-1534.
In general, a link adaptation algorithm makes the decision of whether to maintain or change a current channel coding technique by comparing the estimated channel quality with one or more threshold values. Some amount of hysteresis is typically used in the comparison to avoid ping-pong effects (i.e., repetitive switching between channel coding techniques when the estimated channel quality is about equal to the switching threshold value.)
If the particular link adaptation algorithm is run by the receiver, then signalling is arranged so that the receiver can command the transmitter to use the selected channel coding technique. Otherwise, the signalling is arranged so that the receiver sends the channel quality estimates to the transmitter, which then executes the desired link adaptation algorithm.
A problem arises in these conventional approaches in that, for some cases, an ability to make the channel quality estimation in terms of the C/I or BER may be difficult, inaccurate or even impossible. For example, the receiver may not have sufficient processing power to perform the required measurements. In this case a reduced complexity link adaptation algorithm can be used, such as one that simply counts the numbers of successfully decoded and unsuccessfully decoded packets. However, the packet counting approach, while computationally less complex than other types of channel quality estimation techniques, may result in a problematic link adaptation algorithm. For example, in order to obtain an accurate estimate of a packet error rate (PER) value a significant number of packets must be received, thereby resulting in a slow algorithm. As such, the link adaptation approach based on packet counting will not be capable of rapidly responding to changes in radio channel conditions.
It is a first object and advantage of this invention to provide an improved link adaptation algorithm that overcomes the foregoing and other problems.
It is a further object and advantage of this invention to provide an improved link adaptation algorithm that uses a channel estimation technique based on counting numbers of received and unsuccessfully received packets, and that more quickly adapts to changing channel conditions than conventional packet counting-based techniques.
It is another object and advantage of this invention to provide an improved link adaptation algorithm that uses a channel estimation technique based on counting numbers of received and unsuccessfully received packets, and that improves the speed of the packet counting algorithm by employing statistical testing methods to determine whether a current channel coding technique should be changed.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A method is disclosed for operating a channel coder, as is a channel coder that operates in accordance with the method. The method includes steps of (a) maintaining a first count (N_Number) of transmitted packets and a second count (K_Number) of packets that are erroneously decoded at a receiver; (b) periodically performing a plurality of statistical tests using current values of the first and second counts; and (c) based on a result of the statistical tests, controlling the channel coder to either maintain a current channel coding technique or to switch to another channel coding technique. The step of controlling includes a further step of resetting the first count and the second count.
In a presently preferred embodiment the step of periodically performing a plurality of statistical tests includes steps of (b1), at a crossing point where a first channel coding algorithm (CS-1) and a second channel coding algorithm (CS-2) provide a same net bit rate, assuming as a first hypothesis that a packet error rate (PER) is greater than a PER of CS-1, P1, if CS-1 is currently being used, or assuming as the first hypothesis that the PER is less than a PER of CS-2, P2, if CS-2 is currently being used; (b2) assuming as reference case that N_Number of packets have been transmitted with a constant PER equal to either P1 or P2, depending on the currently used channel coding algorithm CS-1 or CS-2; (b3) determining a first probability (P-value) using the first count and the second count and the constant PER P1 or P2, depending on the currently used channel coding algorithm CS-1 or CS-2; (b4) comparing P-value to a risk level (RL) for determining whether the first hypothesis can be rejected; and (b5) only if the first hypothesis is rejected, changing to the other channel coding algorithm and resetting N_Number and K_Number.
The step of periodically performing a plurality of statistical tests includes further steps of (b6) assuming as a second hypothesis that PER is less than the PER of CS-1, P1, if CS-1 is currently being used, or assuming as the second hypothesis that the PER is greater than the PER of CS-2, P2, if CS-2 is currently being used; (b7) assuming the same reference case as in step (b2); (b8) determining a second probability (P-value) using the first count and the second count and the constant PER P1 or P2, depending on the currently used channel coding algorithm CS-1 or CS-2; (b9) comparing P-value to RL for determining whether the second hypothesis can be rejected; and (b10) only if the second hypothesis is rejected, resetting N_Number and K_Number without changing to the other channel coding algorithm.
In the presently preferred embodiment the step of periodically performing a plurality of statistical tests includes steps of accessing at least one look-up table using the current values of the first and second counts to retrieve probability values (P-values); and comparing the retrieved P-values to a threshold to determine whether the assumed hypothesis should be accepted or rejected.